They were reportedly in talks with multiple studios probably even at the same time, DoubleFine is probably the only one that they were able to close deal before E3. Personally I don’t think that they are slowing down and I don’t think that they should either with Sony now officially announcing that they are going to expand. It’s currently 12 vs 15 although I’m not saying anything crazy like they should go for 20 plus.
Actually with how many multiple team studios Xbox now has they are already at 20+ development teams which compared to a year ago they were sitting at like 5 studios with 5 teams. Most people don't fully see how much of an increase it really is when you count it on a per development team basis instead of per studio.